Method for roasting ores



Sept. 14, 1954 E. KLEPETKO ET AL METHOD FOR ROASTING oREs 4Sheets-Shea?l l Original Filed Aug. 18, 1948 ATTORNEYS Sept. 14, 1954original Filed Aug. 18, 1948 E. KLEPETKO ET AL METHOD FOR ROASTING CRES4 Sheets-Sheet FIG. 2

Sept- 14, l954 E. KLEPETKO ET AL METHOD FOR ROASTING ORES 4 Sheets-Sheet3 Original Filed Aug. `18, 1948 FIG. 7

Sept. 14, 1954 E, KLEPETKQ ET AL 2,689,176

METHOD FOR ROASTING ORES Original Filed Aug. 18l 1948 4 Sheets-Sheet 4FIG. 4 "a INVENTO Rs L'rnesf /f/epf im ATTORNEYS Patented Sept. 14, 1954METHOD FOR ROASTING ORES Ernest Klepetko, Bauer, and Philip de B. Kaye,Salt Lake City, Utah, assignors to Combined Metals Reduction Company,Stockton, Utah, a

corporation of Utah Original application August 18, 1948, Serial No.44,866. Divided and this application October 31, 1950, Serial No.193,096

6 Claims. (Cl. 75--8) This invention relates to roasting ores,concentrates and like metallurgical products, and has for its principalobject the provision of a new and improved roasting method. Thisapplication is a division of our copending application Serial No.44,866, filed August 18, 1948, now Patent No. 2,558,963, granted July 3,1951.

The method of the present invention, which combines the advantages offlash roasting and of hearth roasting, and which comprises a method ofinterrupted iiash roasting, involves causing the particles of the chargebeing roasted to fall freely in a stream of low particle density in aroasting chamber, but only for a small fraction of the height of thechamber in any one uninterrupted drop. Each period of free fall of thecharged particles is interrupted for a length of time that may be variedto suit individual conditions, but which generally does not exceed aboutone-quarter hour, and which in any event is substantially less than thetime during which charge particles are held on any given hearth of aconventional multiple hearth roaster.

It is the purpose of the invention to combine the advantageous featuresof flash roasting, or suspension roasting as it is sometimes called,with the benefits of the bed roasting that proceeds on the hearths ofconventional multiple hearth furnaces, and at the same time to' avoid tothe greatest extent possible the disadvantages inherent in the use ofeither of these methods alone. Flash roasting involves causing a nelydivided charge of oxidizable ore or concentrate to fall freely through aheated oxidizing atmosphere in a suitable chamber. Roasting time is veryshort, being substantially no, longer than the time required for thecharge to fall the height of the chamber. The particles of charge musthe very finely divided or they will be incompletely roasted in thisshort period. Multiple hearth furnace roasting, on the other hand,involves heating a bed of the charge on each hearth of the furnace incontact with the usually oxidizing furnace atmosphere. The charge iscontinually turned over by rakes or rabbles to expose fresh unoxdizedportions to the atmosphere, and is slowly advanced across each hearthand from hearth to hearth by the action of the rabbles.

Flash roasting has the advantage of greater roasting capacity per unitinstallation cost, elimination (in most cases) of external fuelrequirements, and production of roaster gases rich in sulphur dioxideand Well suited to sulphuric acid manufacture. These advantages stemfrom the very short period of intensive oxidation to which each particleof the charge is subjected. Great though these advantages are, they havenot enabled flash roasting to supplant bed roasting in multiple hearthfurnaces, because of the greater exibility and ease of control of thelatter type of roasting. Flash roasting requires a much more finelydivided and more carefully dried charge than hearth roasting, andgenerally results in production of undesirably large quantities of dustin the vflue gases (especially when the ash roaster is stronglyup-drafted to hinder the free fall of the particles and thus increasethe time allowed for roasting to proceed to completion). Control of theroasting temperature is exceedingly diflicult in flash roastingoperations, so that control of the chemistry of the roasting process iscorrespondingly diicult. The nue gases themselves commonly are at such ahigh temperature as to make it difcult to handle them. Because multiplehearth roasting is not subject to these disadvantages it has heretoforebeen favored for most metallurgical roasting operations.

Various proposals have been made heretofore to achieve some of theadvantages of flash roasting in ordinary multiple hearth roastingoperations. For example, it has been recognized that a very substantialpart of the sulphur is burned from a charge of sulphide ore orconcentrate in a multiple hearth roaster during the time that thematerial drops from one hearth to the hearth next below. To capitalizeon this factor, the number of drop holes on the out hearths of multiplehearth furnaces have been increased from 6 per hearth on earlyEvans-Klepetko furnaces to 16 per hearth on furnaces built in the lastdecade. Another proposal, used with some success, has been to remove thecentral hearths from a conventional multiple hearth furnace, so as toform a flash roasting chamber between the top few and bottom fewhearths. To the extent that these expedients have attained theadvantages of dash roasting, however, they have resulted in introducingto a corresponding extent the disadvantages of that method. Very nelydivided and well dried charges have been necesary for optimum results,and control of the flash roasting temperature is not achieved to anyvery great degree.

In the course of an exhaustive study of roasting problems, we havesought to balance idealized roasting conditions against the practicalrequirements of economical commercial practice, and have arrived at aset of criteria which has served as the basis on which we have developedour new roasting method. The dominant elements of the criteria by whichwe have been guided may be summarized as follows:

1. Maximum use should be made of suspension roasting, within the limitsimposed by the desirability of avoiding excessive ne grinding andthorough drying preparatory to roastng.

2. Provision should be made for holding the coarser charge particles inthe roasting operation long enough to effect completion of the roastingreactions, while preferably allowing the finer and so more rapidlyroasted particles to pass through the roasting operation with the leastpossible delay.

3. An adequate supply of air or other oxidizing gas should be availablewherever roasting is proceeding most rapidly, and especially where flashroasting is taking place.

4. Provision should be made for controlling the temperature at whichroasting proceeds.

5. The roasting operation should be conducted so that suitable auxiliaryreagents (e. g. sodium chloride) may be introduced into the charge atoptimum points, when such is desired.

6. The roasting apparatus should be simple, of rugged construction,utilizing known and tried components to the maximum extent possible.

7. Replacement o1' repair of roaster parts subject to most rapid wear ordamage should be accomplished quickly and easily,and to the greatestextent possible without shutting down and cooling the furnace.

The improved metallurgical roasting furnace we have designed on thebasis of these criteria (and which is claimed in our aforesaidapplication Serial No. 44,866) is formed with a cylindrical wall and hastherein a plurality of vertically spaced hearths. Each roasting hearth,however, instead of being of the circular arched refractory designcommon to multiple hearth roasters as heretofore known, comprisesseveral substantially sector-shaped hearth segments (or pallets)arranged in a common plane, with the side edges of each segment spacedlaterally from the adjacent side edges of the neighboring hearthsegments in the same plane. Thus a plurality of substantially radialdrop holes are provided in each hearth, through which material on anyhearth segment may be showered to the hearth below. The hearth segmentsin any one plane, or hearth level, are offset laterally with respect tothe segments in the plane next above to the extent necessary thatmaterial falling vertically through the drop holes will be caught on ahearth segment of the next lower hearth.

Each hearth is provided with rabble arms carrying rabbles that may beset radially of the furnace to push the charge on each hearth segmentprimarily in a circumferential direction toward the drop hole; or thatmay be set at an angle (which alternates in direction of pitch with eachsuccessive rabble arm) to keep turning the charge over as it is advancedthereby circumferentially toward the drop hole.

Provision advantageously is made for delivering a current of airlaterally into contact with the material being roasted as it falls overthe edge of each hearth segment through the drop hole.

The hearth segments themselves are preferably made of heat-resistingalloy steel, generally Without any refractory protective insulation, andare hollow to permit circulation therethrough of a cooling fluid. Eachsector-shaped hearth segment is formed with a mounting ange at its outerarcuate or circumferential edge for fastening it to the cylindricalfurnace wall. As this is the only support that need be provided for thehearth segment, it may be withdrawn through the furnace wall and areplacement may be substituted without it being necessary for workmen toenter the interior of the furnace, or even to discontinue furnaceoperation.

The roasting method of the invention, which is carried out in theabove-described roasting apparatus, involves causing a charge of finelydivided ore or concentratey heated to the roasting temperature, to fallfreely in the form of a thin elongated stream through the oxidizingfurnace atmosphere. The density of the falling particles in the streamis kept low enough so that every particle is in contact with theoxidizing atmosphere. This low particle density is more or lesscharacteristic of the best practice in flash roasting, but, contrary toflash roasting, the method of the invention further involvesinterrupting the free fall of the particles after they have descendedonly a small fraction of the height of the roasting chamber. Suchinterruption occurs, of course, by catching the falling particles on thehearth segment below the drop hole through which they have fallen, andhere they are held long enough to further the roasting of the coarserparticles. Thereafter the material is again caused to fall another smallfraction of the height of the roasting chamber, and again the fall isinterrupted. This sequence of interrupted falls continues until thecharge reaches the bottom of the furnace in roasted condition, resultingin what we term interrupted flash roasting.

The duration of each interruption between successive falls of thecharged material is long enough so that the coarser particles may becompletely roasted, but it is much less than the time during which thecharge is held on each hearth in ordinary multiple hearth roasting.Usually the duration of the interruption is not more than aboutone-quarter hour. In normal operations according to the invention, somepart of the charge is at almost all instants undergoing flash roastingwhile the remainder is undergoing bed roasting on the hearth segments,and all parts of the charge undergo flash roasting and bed roastingalternately.

In a preferred embodiment of the invention,

a stream of air (or oxygen, or oxygen-enriched air) is directedlaterally against the charge as it begins to fall from one hearth levelto the next. In this manner an ample supply of air is provided duringthe period of flash roasting, when the roasting process is proceedingmost intensely. The air stream also can be made to result in some sizeclassification of the charge particles, by delivering it with sufficientforce to blow the finest particles out of the path of substantiallyvertical fall traversed by the coarsest particles. The force of the airstream may be so controlled that the total duration of the interruptionsin the fall of the finest particles as they traverse the roaster issubstantially less than the total duration of the interruptions in thefall of the coarsest particles as they traverse the chamber. In thismanner the fine particles which roast to completion most rapidly, passthrough the furnace in a shorter period of time than the coarse andhence more slowly roasted particles.

Control of the roasting temperature is effected by controlling theduration of the interruptions between falls of the charge particles (i.e. by controlling the number and speed of rotation of the rabble arms),and by controlling the extent of cooling of the hearth segments.Additional control over the roasting temperature is also effected by theconventional expedient of controlling the amount of air admitted tosupport combustion of the roasting charge.

The foregoing is merely an outline of some of the major features of theinvention. These and other features are described in greater detailbelow with reference to the accompanying drawings, which show a roastingfurnace in which the method of the invention can be carried out. In thedrawings,

Fig. 1 is a perspective, with parts broken away, showing the hearthlevels of a furnace in which interrupted flash roasting is proceeding inaocordance with this invention;

Fig. 2 is an elevation of the multiple hearth furnace shown infragmentary section in Fig. 1;

Fig. 3 is a plan of the furnace shown in Fig. 2;

Fig. 4 is a vertical section through the furnace shown in Fig. 2;

Fig. 5 is a horizontal section through the furnace of Fig. 4, takensubstantially along the line 5 5 of Fig. 4;

Fig. 6 is a horizontal section through a hearth segment for the roastingsection of the furnace, taken substantially along the line G-S of Fig.7;

Fig. 7 is a cross section taken substantially along the line 1 1 of Fig.6; and

Fig. 8 is a vertical section taken substantially along the line 8--8 ofFig. 6.

rIhe roasting apparatus shown in the drawings is first described below,and then follows a description of the method of the invention.

Apparatus The general arrangement of the new roasting furnace in theregion wherein interrupted flash roasting takes place is best shown inFig. l. The

furnace comprises a cylindrical steel shell lll havl ing a refractorylining Il and an axial rotatable hollow metallic column I2 provided witha refractory facing I3. A series of hearths at vertically spaced hearthlevels indicated by the arrows A., B and C are provided within thefurnace wall. The hearth at each level A, B, etc., comprises a series ofsector-shaped hearth segments lll. Hearth segments indicated byreference numerals MA, plane of the hearth level A, hearth segmentsindicated by reference numerals lzlB, MB', etc., are arranged in thecommon plane of the next lower hearth level B, etc. The radial sideedges i5 (herein called the forward side edges) of each hearth segmentare spaced laterally from the adjacent radial side edges l (the rearwardside edges) of the neighboring hearth segments in the same common planeor hearth level. Thus between each adjacent pair of hearth segments in agiven hearth level is a fairly wide radial opening (such, for example,as the opening between the forward side edge l5 of hearth segment MA andthe rearward side edge it? of the hearth segment MA) which serves as adrop hole through which ore or concentrate being roasted may fall fromone hearth level to that next below.

It will be noted that the hearth segments in any given hearth level arestaggered laterally with respect to the hearth segments in the levelsnext above and next below. For example, the hearth segments 14B, MB',etc., in the B hearth level are not arranged directly below the hearthsegments MA, I4A, etc., in the A hearth level MA', etc., are arranged inthe common 2l into two compartments 22 and 23.

next above, but are offset laterally with respect thereto so as tounderlie the radial drop holes between these latter hearth segments.Thus each hearth segment is in position to receive material fallingvertically over the forward edge i5 of a hearth segment in the levelnext above it.

Rabble arms Il project laterally from the axial column l2 at each hearthlevel. Each rabble arm carries a series of rabbles I8. The rabbles,contrary to usual multiple hearth furnace practice, are shown in Fig. 1arranged parallel to the radially extending rabble arms, and they serveto push ore or concentrate being roasted on any of the hearth segmentscircumferentially toward the forward side edge l5, over which it mayfall through the wide radial drop hole to the hearth below. In view ofthe arrangement and action of the rabbles i3, they might more accuratelybe termed pushers, but we prefer to use the more conventional termrabbles to identify them. The rabbles i8 may, however, be set at anangle to the axis of the rabble arm. If this is done the pitch of theangle should alternate from one rabble arm to the next, so that whilethe rabbles then will keep turning over the charge on each hearthsegment, they will not, in net effect, tend to move it radially in orout along the hearth, but will move it circumferentially to the droplhole.

As shown in Figs. 1, 6, 7 and 3, each hearth segment Ui is a hollowmetallic sector-shaped elementy having top and bottom walls i9 and 2ejoined together by the side edges and ifi. such segment is dividedinteriorly by a partition The partition 2l docs not extend all the wayto the apex end 2d of the hearth segment, but instead terminates at apoint 25 short of the apex, so to provide an opening 26 forcommunication be'- tween the lower and upper compartments and 23 withinthe segment.

The arcuate or circumferential end portion El of each hearth segment isenlarged to provide cooling fluid inlet and exhaust passageways and 29.The inlet passageway 28 communicates with the compartment 22 below thepartition 2 i, and the exhaust passageway 29 communicates with thecompartment 23 above the partition. The inlet and exhaust paesagewaysadvantageously are formed integrally with the hearth segment, andintegrally also with mounting flanges 3i] by which the segment isfastened in place in the furnace. Each hearth segment extends thro-ugh acircumferentially-extending slot in the steel shell lil of the furnace,and is secured in place by bolting the mounting @il to the furnaceshell. This is the only support necessary to retain the hearth segmentin place, and to enable it to carry its own weight and that of thecharge which falls upon it. Consequently it is a simple matter to removecr insert any particular hearth segment, without entering the interiorof the furnace or even discontinuing its operation. To facilitateinserting a hearth f segment into the furnace, or removing it therefrom,the top and bottom walls of the cooling iiuid inlet and exhaustpassageways are tapered as indicated at 5I (Fig. 8).

To keep the hearth segments from overheating during operation of thefurnace, cooling air (or other cooling fluid) is admitted to theinterior of each segment through a supply pipe 32 communicating with theinlet passageway 28. The air flows thence through the lower compartment22 to the opening 26 and back through the upper compartment 23 to theexhaust passageway 29, from which it escapes through an exhaust pipe 33.The longitudinal partition 2l thus serves primarily as a baflle fordirecting the cooling air to the apex 24 of the segment before itescapes. In place of using a horizontal longitudinal partition 2| (asshown in the drawings) for this purpose, a vertical longitudinalpartition can be used instead.

A vertical longitudinal partition 34 (Figs. 3 and 6) is arrangedinteriorly of each roasting hearth segment near its forward side edgeI5, to form a combustion air inlet manifold 35. The forward side edge lis itself formed with a series of openings 35, which serve the purposeof directing a stream of air laterally against the charge being roastedas it drops over the forward side of the hearth segment to the hearthnext below. Combustion air is admitted to the manifold 35 through inletpipe 31 controlled by a valve 38 (Fig. 6) and may be taken withadvantage, as preheated air, from the exhaust pipe 33 through whichspent cooling air escapes from the outlet passageway 29. If some othermedium than air is used for cooling the hearth segments, the valvedinlet pipe 31 is connected to some other convenient source of air orother oxidizing gas.

A substantially complete roasting furnace, in which interrupted flashroasting hearths according to Fig. l are incorporated is shown in Figs.2 to 5. The furnace is shown as having six vertically spaced hearths 49to 45, but additional hearths may be provided if desired. The uppermosthearth 40 receives the charge and serves as a preliminary drying hearth.The second hearth 4i is a drying and distributing hearth, and isarranged to insure proper distribution of the charge on the hearthsegments of the rst interrupted nash roasting hearth 42 (shown as thethird hearth of the furnace). Additional interrupted flash roastinghearths, in any desired number, are arranged below in the mannerdescribed above with reference to Fig. 1 (only two such additionalhearths 43 and 44 are shown in Fig. 4) The lowermost hearth 45, whichforms the floor of the furnace, is a finishing hearth from which theroasted product is withdrawn.

The two upper drying hearths 40 and 4i are full circular hearths. Theymay be of the usual arched refractory brick construction, butadvantageously they are made of metallic segments of substantially thesame construction shown in Figs. 6 to 8, but without the longitudinalpartition 34 as there is here no need for a combustion air manifold 35or openings 36. The upper drying hearths may be heated rather thancooled to facilitate drying of the charge.

Ground ore or concentrate is delivered to the upper drying hearth 40 andis spread evenly on it by a charge distributor 45 (Fig. 4). The chargedistributor receives the material to be roasted from a hopper 41 towhich it is delivered through a feed pipe 48. A screw conveyor 49 insidethe charge distributor moves charge radially out from the hopper 41,discharging it through distributing outlets 50 along the radius of thehearth. At the same time the distributor is moved slowly in a circularpath about the axis of the central column, thus spreading the chargeevenly over the upper hearth 40. A furnace cover 5|, on which the chargedistributor is mounted, is itself mounted on wheels 52 riding on acircular rail 53 at the top of the furnace shell IE), so as to permitthe circular travel of the distributor. A motor 54 drives thedistributor and cover in their circular path, and also operates the wormconveyor 49. A seal plate 55 extending up from the furnace shellprovides a fairly gas-tight joint between the cover 5l and the main bodyof the furnace.

The upper hearth 40 is provided with a series of radial drop holes 5S(Fig. 5) and is served by rabbles 51, 51 carried on rabble arms 58, 55which in turn are mounted on the central column I2 of the furnace. Therabbles are pitched at an angle to the axis of the rabble arms, but thedirection in which the rabbles 51 on one arm 58 are pitched is oppositeto that of the rabbles 51 on the other arm 58. The net result of thisarrangement is that the rabbles do not move 'the charge substantiallyeither in or out along the radius of the furnace, but keep turning itover and working it circumferentially toward the drop holes.

The second drying hearth 4I is of essentially the same construction asthe upper drying hearth 4t. It is provided with radial drop holes 59,and is served by rabbles '59, 60 arranged in the same way on rabble arms6|, 5I as the rabbles 51, 51' serving the upper hearth. However, theradial drop holes 59 of the second hearth are located so as not to liedirectly under the drop holes 55 of the upper hearth (otherwise chargefalling from the upper hearth 40 would not be caught on the secondhearth il The next three hearths 42, 43 and 44 shown in Fig. 4 areinterrupted flash roasting hearths constructed and arranged as describedabove in cennection with Fig. 1. The hearth segments of the upper ofthese hearths 42 are so located that their rearward edge portionsunderlie the radial drop holes 59 of the second drying hearth lli, so asto catch the charge as it falls from the lowermost drying hearth. Theinterrupted flash roasting hearths 42 to 44 are served by rabbles l5carried on rabble arms l1, as previously described, and these rabblesmay if desired be set parallel to the rabble arm so as to act more likepushers for moving charge directly across the hearth segments to theradial drop holes between segments. However, the roasting hearth rabblesi8 mai`T alternatively be arranged in the same manner as the rabbles 51,5l and 5G, Gil serving 'the drying hearths, so as to move the chargemore slowly across the roasting hearth segments and turn it over moreeffectively.

The lowermost hearth 45 is in effect the bottom of the furnace, and isadvantageously of refractory brick construction. It is served by rabblearms 52 carrying rabbles 53 which are set at an angle to move the chargeoutwardly from the center of the furnace to the periphery, in 'theconventional manner of out hearths of ordinary multiple hearth furnaces.The roasted product is discharged from the bottom hearth 5:5 through oneor more outlets 54 at furnace periphery.

The central column l2 on which the rabble arms l1, 58, 5l and 52 aremounted is of more or less conventional construction. It isadvantageously in the form of a large hollow metallic shaft, providedwith hollow rabble arm socket rings at each hearth level. The column AEssupported on a heavy roller bearing t5 at its base, and is provided withthe usual ring gear (il, through which the column may be slowly rotatedby a pinion 58 and associated drive mechanism. A bearing 59 mounted in asupporting frame "55 (which may in turn be supported from the furnaceshell) is provided near the top of the column !2 to hold it upright inits axial position within the furnace.

The arrangement for cooling the rabble arms is conventional, but isdescribed below in some detail for the sake of clarity. A cooling fluidinlet pipe 'Il communicates with the interior of the column through thecentral opening of the supporting bearing Se. The interior of the columnis divided into two compartments lil and i3 by a longitudinal partition'lf-3. Cross partitions l5 at the top and bottom of the longitudinalpartition 'lll prevent direct communication between the compartments l2and 'i3 on opposite sides of the longitudinal partition. Cooling airdelivered through the inlet conduit l! enters only the compartment E2 onone side of the partition it.

Each of the rabble arms Il etc. below the upper cross partition. 'i5 ishollow and is provided interiorly with a pipe 'i6 or it which extendsfrom the inner end of the rabble arm almost to its outer end. Pipes 'itin rabble arms mounted on one side of the partition lll communicate withone of the compartments 42, and pipes 'li-5 in rabble arms on theopposite side of the partition cornmunicate with the other compartment'13. Each annular rabble arm socket ring G5 is hollow, deiining anannular cooling medium passage ll, and the interiors of the rabble armsil etc., outside the pipes lil, communicate with this passage throughports 'it and lt.

Cooling air or other cooling medium entering the compe. tment 'itthrough the inlet conduit ll can new only into the pipes it, and thenfrom the ends of these pipes must flow back through the hollow raboearms and through the ports i8 into the annular passages 'il in therabble arm socket rings. The air then flows through these passages toand through the ports i8 and into the interiors cf the rabble armsmounted on the opposite side of the partition 1li. Thence the flow isthrough the rabble arms and .back through the pipes into the compartmenti3.

The spent cooling air passes from the compartu ment it through anexhaust conduit 'it at the top of the furnace. A liquid cup seal 8f4 isprovided at junction of the exhaust condiut 'iii with the column if. Adamper may be provided in this conduit near the top of the furnace, ifdesired., to control the rate of iiow of cooling air through rabblearms.

In order to prevent overheating of the metallic hearth segments and toprovide for control over the temperature prevailing during roasting, acooling fluid supply header di (Figs. 2 and 3) connected to bustle pipesiii. is arranged to deliver coolinCr fluid to each of the hearthsegments of the interrupted flash roasting hearth (i2, d3, riffs, etc.,and to the lowemost drying hearth il (since the bottom of this hearth isvexposed. tol the roasting zone of the furnace, it requires effectivecooling). The supply pipes 32 of the hearth segments are connected tothis bustle pipe. The cooling fluid entering each hearth segment flowsthrough the compartment 22 on one side of the partition 2l and backthrough the compartment 23 on the other side to the cooling iiuidexhaust passageway 253 (as above described in connection with Figs. 6 to8) whence it flows through the exhaust pipes S3 to a second set ofbustle pipes 83 connected to an exhaust header 3G.

desired, provision may be made to utilize the heated spent cooling airfrom the interrupted iiash roasting hearth segments for preheating anddrying the incoming charge on one or more of the upper drying hearthslili. In the arrangement shown in Fig. 2 for this purpose, a pair ofbustle pipes and 86 are connected to` the exhaust header `8d below andabove a damper 81, respectively. The supply and exhaust pipes 32 and 33of the hearth segments are connected to these bustle pipes (here thepipes 32 that are exhaust pipes on the roaster hearth segments are usedas supply pipes, and vice versa, for effective heating and drying of thecharge). Any desired proportion of the hot spent cooling air from theroasting section of the furnace may be :by-passed from the exhaustheader through the segments of the drying hearth 4U by appropriateadjustment of the damper 81.

To provide for withdrawal of roaster combustion gases, one or morecombustion gas outlet flues Se (preferably at least one such flue serveseach hearth) connects the interior of the furnace with a combustion gasexhaust stack t9. Dampers et in the fiues 38 are provided for regulatingthe draft through the furnace. A conventional gate valve at the bottomof the stack is provided for removal of flue dust that settles out inthe stack.

At least one access door ti is provided at each hearth level, both topermit observation of and access to the interior of the furnace, and toadmit combustion air. Any desired number of fuel burners 92 may also beprovided to heat the charge to the roasting temperature when startingthe furnace, and also to keep the charge at the roasting temperature ifit does not contain enough sulphur to roast autogenously.

Downwardly sloping ,annular protective shields St (Fig. 4) are securedto the rotatable central column l2 at each hearth level to preventparticles of charge from. falling between the refractory facing lof therotating Central column and the adjacent edges of the stationary hearthsegments. vThe hearth segments may be provided at their inner ends withan upstanding lip 9d which extends up behind the cooperating shield 93.

A feature of importance of the new furnace, resulting from the use ofall-metallic sectorshaped hearth segments fastened only to the furnaceshell is the ease with which a hearth segment may be removed for repairsand may be replaced without shutting down the furnace. The hearthsegment is simply withdrawn radially through the circumferentiallyextending slot in the furnace wall in which it is mounted, as indicatedin dotted lines at ld', Fig. 3. Each of the hearth segments normally isheld in place solely by bolts 95 passing through holes et in itsmounting flanges 36 and threaded into tapped holes in the steel shell H3of the furnace. No fastening means are needed inside the furnace forholding the hearth segment in place. The flange mounting is adequate forsupporting both the weight of the hearth segment itself and any chargeupon it. Accordingly, by simply removing the bolts 95, the entire hearthsegment may be withdrawn from the furnace and another similan hearthsegment may be inserted and fastened in place. If there is an adequatedraft through the stack 8d, this operation may be performed even whileroasting is going on within the furnace.

Method The roasting method of the invention is carried out in thefurnace described above substantially as follows: Quite `finely crushedore, or a notation Iconcentrate of the usual degree of fineness, such asa zinc sulphide or copper sulphide concentrate, is introduced throughthe feed pipe 48 and is spread even over the top drying hearth 40 by thedistributor 4B. The rabbles l and 5l keep turning it over and moving itgradually to the radial drop holes 56, whence it falls to the seconddrying hearth 4I. Again it is repeatedly turned over, by the rabbles 6Uand 6D', and is gradually moved to the radial drop holes 59, throughwhich it falls to the hearth segments (or pallets) of the rst roastinghearth level At2. While the charge is being moved across the dryinghearth 40 it is heated by the hot spent cooling air delivered to theinterior of the hollow metallic segments of this hearth from theroasting hearths below; and thereby it is dried sufficiently forroasting to begin.

The charge falls in an elongated stream through the radial drop holes 5gand is collected quite uniformly along the radial length of theunderlying segments of the first interrupted flash roasting hearth 112.As it falls thereto, it is heated by the hot atmosphere in the roastingsection of the furnace to the temperature at which roasting begins.Assuming the charge to be a sulphide mineral capable of autogenousroasting-i. e. able to sustain combustionsome roasting will be initiatedas the charge falls from the last drying hearth, and the roastingreaction will spread through the charge as it collects on the segmentsof the rst interrupted flash roasting hearth.

Quite shortly after a portion of the charge has been deposited on asegment of the first interrupted flash roasting hearth, it will bepushed therefrom over the forward side edge by the aotion of the rabblesi8, and flash roasting will ensue as it showers tc a segment of the nextlower hearth level. Bed roasting then proceeds for a further shortperiod of time, until the charge is again pushed by the rabbles over theforward side edge of the hearth segment, to fall in a flash roastingenvironment to the next hearth. These periods of flash roasting withintervening periods of bed roasting on the hearth segments follow oneanother until the charge finally arrives at the bottom hearth 65 of thefurnace. Here the rabbles move the charge to the outer periphery of thefurnace, where it drops out through the discharge conduits Sri.

The combustion gases formed during roasting of the charge pass outthrough the ues 88 and escape through the stack 8S. With one or moreflues 8S serving each hearth level, the nature and extent of the draftthrough the furnace is readily controlled by suitable regulation of thedamper 9S in each flue.

The movement of the charge through the roasting section of the furnaceis most clearly visualized by reference to Fig. 1. Once the charge hasbeen distributed uniformly along the radial length of the hearthsegments in the first roasting hearth level (say hearth level A), it ismoved quite uniformly by the rabbles over the forward side edges of thesegments and falls in a thin elongated stream of low particle density tothe segments of the hearth next below. It is desirable of course thatthe particle density in. the stream of charge falling from one hearth tothe next be sufciently small so that every particle is in contact withthe oxidizing atmosphere of the furnace during its fall. This isnecessary to insure optimum ash roasting. Conventional flash roastingfurnaces are, of course, designed to shower the charge through theroasting chamber in a stream of low particle density; but in theconventional multiple hearth furnace, the particle density of the streamof charge falling through the drop holes is so great that only veryincomplete hash roasting can take place-the high particle density of thefalling charge in such furnaces locally depletes the oxidizingconstituents of the furnace atmosphere to such an extent that many ofthe particles fall all or part of the distance to the next hearth belowwithout coming in contact with suiiicient oxidizing gas to beeffectively roasted.

Two results of major importance ensue from the above-described method ofinterrupted flash roasting. The first such result is that the total timeof flash roasting, in a furnace of given height, is very substantiallyincreased as compared with a conventional fiash roasting operation. Thisfollows from the fact that as a particle falls it accelerates, andtraverses a given distance much more rapidly toward the. end of its fallthan toward the beginning. For example, a freely falling body takes 1.12seconds to fall vertically 20 feet; but if the fall is interrupted at,say, flfoot intervals, the falling time to traverse a total distance of20 feet is increased to 2.5 seconds. This is precisely what occurs inthe new roasting method-neglecting the hindering eiect of an updraftthrough the furnace, the time of free fall for each particle of thecharge traversing the roasting section of the furnace is substantiallygreater than in a conventional flash roasting operation in a roastingchamber of equal height. At the same time the low particle density inthe owing stream of charge that is characteristic of flash roasting butnot of conventional multiple hearth roasting is achieved.

The second major result is that during the periods the charge is held onthe segments of the several hearths, the larger particles are given timeto be effectively roasted. As pointed out above, ordinary flash roastingoperations require a very finely divided charge in order that eachparticle may be roasted completely in the short period of time allowed.In the new interrupted hash roasting method, however, substantiallylarger particles in the charge are permissible, because the time duringwhich the particles are held in a roasting environment on the hearthsegments is very much greater than can be achieved in ordinary iiashroasting. On the other hand, the time of retention on the hearthsegments is much shorter than in conventional multiple hearth roasting,so that the charge may be advanced much more rapidly through the furnaceby the new method.

With rabbles i8 set parallel to the rabble arm I1 and acting as pushers,the time of retention of the charge at each hearth level is determinedby the number of rabble arms and by the speed of rotation of the centralshaft. An increase in the number of rabble arms at any given hearthlevel for a given speed of rotation of the shaft decreases the timeinterval between two sucn cessive passes of the rabbles over a givenhearth segment; and, of course, such time interval decreases with anincrease in the speed of rotation of the central column. Since pushertype rabbles wipe a hearth segment substantially clean each time theypass over it, the retention time can be controlled by the number ofrabble arms used and their speed of rotation. The same is for the mostpart true if the rabbles are inclined at an angle to the rabble arms,except that in this case the rabbles do not wipe the hearth segmentclean with each pass over it. Consequently with inclined rabbles theretention time is considerably longer than with 13 pusher type rabbles,but is still much shorter than in a conventional multiple hearth furnacebecause of the provision of the plurality of radial drop holes.

In general, it is advantageous to retain the roasting charge at eachhearth level for a period of time that is long in relation to the lengthof time required for free fall through the height of the furnace, butwhich, in general, does not exceed about one-quarter hour. Aquarter-hour is a Iconsiderably shorter time interval than that duringwhich the charge is held on each hearth of a conventional multiplehearth roaster (which is generally about an hour or so). lt is a periodof time that is ample for effective roasting of charge particles muchcoarser than can be treated successfully in conventional flash roastingoperations; and yet it is so short in relation to the time intervalinvolved in multiple hearth roasting as to greatly increase the rate atwhich a charge may be roasted.

As indicated above, it is very desirable to direct a stream of air (orother oxidizing gas) laterally against the charge, as it falls from onehearth level to the next. In the apparatus shown in Fig. l, a stream ofair is delivered. into contact with the charge through the openings 36as it falls over the forward side edges l of each hearth segment.Several advantageous results are accomplished in this manner. One resultis that an ample supply of oxidizing gas is brought into contact withthe charge at just the point where it is required-that is, at the pointwhere flash roasting commences or resumes. A second result is that thestream of air, if delivered with sufficient force, blows the :linerparticles of the falling charge farther out from the forward edge of thehearth segment than the coarser particles,

thus forcing a decreased particle density in the v falling stream ofcharge. In consequence of this second result, the finer particles fallto the hearth segment next below at a point nearer its forward edge thando the coarser particles. Thus these nner particles, which require aminimum of bed roasting, are in position to be among the first to berabbled to the next drop hole. If inclined rather than "pushen typerabbles are used, the fines falling toward the forward edge of thehearth segment may be rabbled to the drop hole with the very next passof a rabble arm, while the coarser particles may not be advanced to thedrop hole until after several passes of; a rabble arm over the hearthsegment. Still greater size classification of the particles results ifthe stream of air issues from the openings 3B with suicient force toblow the finest particles in the charge far enough forward to miss thehearth segment immediately below, and to continue falling at least tothe second hearth below.

To illustrate the foregoing, consider a charge falling over the forwardedge I5 of a hearth segment ilA in the A hearth level of Fig. l. Shouldthe charge fall vertically, it would be caught near the rearward edge ofa hearth segment HiB in the B hearth level. However, a stream of airissuing from the openings 36 in the upper hearth segment lliA will blowsome of the fines forward to be caught near the forward edge of thelower hearth segment llB; and if the force of the air stream issuiiiciently great, only the coarser particles will be caught at all onthis hearth segment iflB, while the finest particles will be blownforward sufficiently to fall through the radial drop hole between thehearth segments MB and MB in the B hearth level to the hearth segment|4C two hearth levels below. Thus in this latter case the finestparticles miss the B hearth level altogether-their rfall is interruptedat the C hearth level, substantially lower in the furnace than the Bhearth level where the fall of the coarse particles is interrupted. Ineither case, however, finest particles, which roast most quickly, maythus be made to traverse the furnace in a substantially shorter lengthof time than the coarser particles, which require a longer time forroasting.

Effective control 'of temperature is important in order to secure mostsatisfactory roasting results. It is virtually impossible to control thetemperature of a conventional flash roasting operation, particularly atdifferent levels in the flash roasting zone. Co-nsiderably bettersuccess in this respect is attainable in multiple hearth roastingoperations, but often at the expense of decreasing the capacity of theroasting furnace. This is because the temperature vcontrol in suchfurnaces is generally achieved by controlling the rate of speed of therabble arms and the rate at which air is admitted to the furnace.Temperature control in these manners directly affects the rate at whichthe charge is or can be passed through the furnace. Furthermore, thehigh reaction temperature attainable in properly ccntrolled flashroasting cannot be reached in conventional multiple hearth roasting.

In the roasting operation of the invention, the same temperaturecontrols used in ordinary multiple hearth roasting are available, but inaddition the temperature may further be 'controlled by regulating theflow of cooling air or other cooling medium through the metallic hearthsegments. Temperature vcontrol in this latter manner is often moreprecise than by the methods available in ordinary multiple hearthroasting operation, and in addition permits different temperatures to bemaintained, within limits, at difference hearth levels in the roastingsection of the furnace. For example, in roasting an iron-bearing zincsulphide concentrate, it is generally desirable to avoid the formationof complex zinciron compounds. Such compounds will form if zinc sulphideis in contact with iron sulphide in a bed of charge heated to theroasting temperature of zinc sulphide. However, if the roastingoperation is initiated at a relatively low temperature, at which theiron sulphide oxidizes readily but at which the zinc sulphide is notsignificantly affected, then subsequently the Zinc sulphide may beroasted at the higher temperature required for its oxidation withoutformation of undesirable complexes. The roasting apparatus and method ofthe invention permit of achieving such preferential initial roasting ofone component of a charge, before roasting of the other component beginsto any substantial extent, because sufriciently close control of lthetemperature of the roasting charge at each of the different `hearthlevels may be attained.

The rate at which iiash roasting proceeds relative to the rate of bedroasting also may be controlled within limits by controlling the rate atwhich air is admitted through the openings 35 at the forward edges ofthe roasting hearth segments. This serves also as an added means forcontrolling the temperature in the furnace, for the relatively hightemperature developed in flash roasting is transferred by the furnaceatmosphere to all parts of the furnace.

Roasting operations sometimes are carried out in conjunction with anadded solid reagent, as

for example when sodium chloride is added to a roasting charge to eiiecta chloridizing roast. It is generally not feasible to incorporate anadded reagent in the charge delivered to a suspension roastingoperation, because the time interval of the roasting operation is tooshort for a plurality of diiferent reactions, one dependent upon theother, to proceed to substantial completion. 1n the process of theinvention, however, it is thoroughly feasible to incorporate a solidreagent with the charge, either when the charge is rst delivered to thefurnace, or at some point in the roasting section of the furnace belowthat at which roasting first begins, in the same manner as is possiblein ordinary multiple hearth roasting. In the method of the invention,the reactions involving the added reagent usually proceed more rapidlythan in ordinary multiple hearth roasting, because they may in partproceed at the relatively higher temperatures occui-ring during theintervals of ash roasting.

We claim:

1. In the roasting of ores and concentrates, the improvement whichcomprises causing a stream of such material in finely divided form andheated to its roasting temperature to fall over the edge of a hearth andthrough a drop hole, and initially contacting a fresh stream oflaterally-directed air with the falling stream of charge substantiallyimmediately as it passes over the edge of the hearth.

2. The method of roasting ores and concentrates which comprises causinga finely crushed charge of such material heated to its roastingtemperature to fall in the form of a thin elongated stream through anoxidizing atmosphere, directing a stream of air laterally against thefalling stream of charge in such direction and with sufficient force toblow the most finely divided charge particles through a substantiallateral distance in the direction of their movement through the roastingzone but with less than suiiicient force to deect the coarsest of thecharge particles substantially from a vertical path of fall,interrupting the fall of at least the coarsest particles after they havefallen a short distance, and thereafter causing the particles whose fallhas been interrupted to fall a further short distance through theoxidizing atmosphere.

3. The method of roasting ores and concentrates which comprises causinga nely crushed charge of such material heated to its roastingtemperature to fall in the form of a thin elongated stream through anoxidizing atmosphere, directing a stream of air laterally against thefalling stream of charge in such direction and with sufficient force toblow the most nely divided charge particles through a substantiallateral distance in the direction of their movement through the roastingzone but with less than sufficient force to deflect the coarsest of thecharge particles substantially from a vertical path of fall,interrupting the fall of said coarsest particles after they have fallena relatively short vertical distance, and interrupting the fall of saidmost finely divided particles only after they have fallen a relativelysubstantially greater vertical distance.

4. The method of roasting ores and concentrates which comprisesintroducing a finely crushed charge of such material into the upperportion of a roasting chamber, heating the charge to its roastingtemperature, causing the heated charge to fall in the form of a thinstream across a laterally directed stream of air, said stream of airbeing directed in such direction and of suincient force to deflect themost finely divided charge particles through a substantial lateraldistance in the direction of their movement through the roasting zonebut of less than sunlcient force to deflect the coarsest chargeparticles substantially from a vertical path of fall, interrupting thefall of at least the coarsest particles after they have descended only asmall fraction of the height of the roasting chamber, and repeatedlycausing the charge particles to fall in the manner just defined across alaterally directed stream of air after each interruption of their falluntil they have descended to the bottom of the chamber, the totalduration of the interruptions in the fall of the finest particles asthey traverse the chamber being substantially less than the totalduration of the interruption, in the fall of the coarsest particles asthey traverse the chamber, whereby the most finely divided particlespass through the roasting chamber substantially more quickly than thecoarsest particles.

5. The method of roasting ores and concentrates which comprisesintroducing a finely divided charge of such material into the upperportion of a roasting chamber of substantially circular cross-section,heating the charge to its roasting temperature and hash roasting theheated material by causing it to flow freely in the form of an elongatedstream through an oxidizing atmosphere, the density of falling particlesin the stream being sufliciently small so that every particle is incontact with the oxidizing atmosphere as it falls therethrough,interrupting the free fall of the material before it has descended morethan a small fraction of the height of the roasting chamber,bed-roasting the material by retaining it for an appreciable period oftime at the point at which its free fall was interrupted, moving itthrough only a small sector of said roasting chamber and then causingthe material to fall freely in the manner just defined through anothersmall fraction of the height of the roasting chamber, again interruptingits fall and retaining it there for an appreciable period of time, againmoving it through only a relatively small sector of the said roastingchamber whereby it is once more caused to fall freely in the mannerdefined through a further small fraction of the height of the chamber,initially contacting a fresh stream of a laterally-directed oxidizinggas with the falling streams of the ore after each bed roastingoperation substantially immediately as such streams begin their freefall, and repeating this sequence of operations until the material hasbeen roasted to the desired extent.

6. The method of roasting according to claim 5, characterized in that anauxiliary reagent for reacting with the charge during the roastingoperation is added to the charge.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 522,421 Jones July 3, 1894 677,263 Pape et al. June 25, 1901'799,696 Ward Sept. 19, 1905 879,842 Swart Feb. 18, 1908 1,671,395 BairdMay 29, 1928 2,421,542 Connolly June 3, 1947 2,558,963 Klepetko et al.July 3, 1951

1. IN THE ROASTING OF ORES AND CONCENTRATES, THE IMPROVEMENT WHICHCOMPRISES CAUSING A STREAM OF SUCH MATERIAL IN FINELY DIVIDED FORM ANDHEATED TO ITS ROASTING TEMPERATURE TO FALL OVER THE EDGE OF A HEARTH ANDTHROUGH A DROP HOLE, AND INITIALLY CONTACTING A FRESH STREAM OFLATERALLY-DIRECTED AIR WITH THE FALLING STREAM OF CHARGE SUBSTANTIALLYIMMEDIATELY AS IT PASSES OVER THE EDGE OF THE HEARTH.